The present invention relates generally to optical laser telescopes, and more specifically to a pointer/tracker apparatus for optically phasing an array of multiple telescopes for use as a laser transmitter, and providing off-axis point of the array.
A synthetic aperture is formed when separate optical systems are combined to function as a single larger aperture. When an aperture is synthesized, independent optical systems are phased to form a common image field with resolution determined by the maximum dimension of the array and therefore exceeding that produced by any single element. By optically phasing an array of multiple telescopes, a synthetic aperture is formed which can achieve the performance of an equivalent sized, single laser transmitter.
Phased arrays are currently in use. The successful application of phasing an array of multiple telescopes into a synthetic aperture is the subject of the patent filed by Janet S. Fender et al, entitled "Optically Phased Laser Transmitter", in Air Force Invention No. 16,653 and U.S. Pat. No. 4,639,586, the disclosure of which is incorporated herein by reference. The apparatus of the Fender patent is a laser transmitter which optically phases the output of an array of multiple optical laser telescopes to achieve the performance of a single laser transmitter of equivalent size.
The Fender apparatus performs phase matching between pairs of laser beams using an array containing at least two optical telescopes which become useable as a laser transmitter when combined with an optical phase matching system consisting of: a collecting telescope, a detector array, two fold mirrors, analog-to-digital converter, microprocessor, and two sets of correcting mirrors.
The two optical telescopes are adjacent to each other, and transmit two separate outgoing laser beams which require phase matching. The original source of the two outgoing beams may be either: a single laser beam, which has been divided (monochromatic); or two separately transmitted polychromatic laser beams.
The sampling mirror is able to intercept samples of outgoing laser beams from the edges of both telescopes and focus them through the two fold mirrors to the detector array.
The detector array may be either a line scan or an area charge coupled device (CCD), which reads out the interference pattern. The microprocessor receives the interference pattern between samples of pairs of transmitted laser beams from the CCD camera, then performs a calculation of the difference in optical path lengths between the two beams which allows the laser transmitter to match the phase of the outgoing beams by adjusting the optical path lengths with the correcting mirrors.
Note that this application incorporates by reference a patent filed by Richard A. Carreras entitled "Microcomputer Controlled Image Processor" and described in Air Force Invention No. 16,652, and U.S. Pat. No. 4,689,758. Mr. Carreras provides a detailed apparatus and technique to calculate the optical phase difference for two signals from an interference pattern provided by the CCD camera. The Carreras system has been successfully used with the Fender et al system to determine the difference between two beams using one of a number of phase estimating algorithms.
Once an accurate estimate of the phase difference is determined, the estimated error is used to adjust the phase by the correcting mirrors which adjust the optical path lengths of the two outgoing beams.
The original tracker designed for the phased array implemented a concept of the common physical element. The theory was that low frequency drifts in electronics, optics, and mechanical components caused distortion of the point spread function in the far field pattern. It was felt that if the optics and electronics were common to all three telescopes that the long term distortion effects could be controlled. That was true to a point, but the tracker offered limited capability. Off axis pointing presented a new problem that was beyond the capability of the tracker. Additionally, the tracker required complicated electronic circuitry that was difficult to align and maintain. It also caused ambiguities and instabilities in the tilt control loops. Through experimentation it was found that a common element was needed for improved tilt control of each synthetic aperture As a result of this need, the invention of Mr. Carreras et al entitled "Synthetic Aperture Multi-Telescope Tracker Apparatus" was developed. This system is the subject of U.S. Pat. No. 4,667,090, the disclosure of which is incorporated herein by reference. While the Carreras et al system provides improved tilt control, it offers limited flexibility, particularly when all telescopes are forced to dynamically track a reference.
Another system which may be used for rapid optical phasing of optical devices using white light interferometry is disclosed in the patent application of Mark Baciak entitled "Method and Apparatus for Rapid Optical Phasing", U.S. application Ser. No. 857,621, the disclosure of which is incorporated herein by reference. The system of the Baciak reference includes a white light source, a beam splitter, a scanning mirror and a stationary reflective surface. The scanning mirror is moved in piston while reflected light is observed by an electro-optic sensor at a receiver. A nulled pattern has been previously established at the receiver. An electrical signal from the electro-optic sensor is mounted at an amplification and recording means to identify variations in reflected light intensity that are caused by white light fringes. White light fringes only occur at zero optical difference, i.e. when the scanning mirror and stationary reflective surface are in phase.
The Baciak reference is important because it discloses a method phasing a plurality of optical devices. The method comprises transmitting light through a beam splitter to a reference mirror and an optical device to be phased. Light reflected from the reference mirror and the optical device are recombined at the beam splitter and directed to a receiver. Light at the receiver is converted by an electro-optical sensor to an electrical signal which is transmitted to a detection and amplification means. The optical device is then moved along the path of light directed to it (i.e., in piston) from the beam splitter until a variation in light intensity at the receiver indicates that there is zero optical difference between the reference mirror and the optical device being tested. The first optical device tested can then be considered a reference and this process repeated with other optical devices to be phased.
Another exemplary phase matching system is disclosed in U.S. Pat. No. 4,600,308 by T. Waite entitled "Phase-Matching Arrayed Telescopes with a Corner-Cube-Budge Metering Rod" the disclosure of which is incorporated herein by reference. The Waite system resembles that of Janet Fender in that it uses an optical trombone to optically adjust the phase of adjacent telescopes through changes in optical path lengths.
All of the references cited above demonstrate the interest in a relatively emerging technology and phased arrays of optical telescopes. The current trend of developments resemble a similar development in phased array radar systems experienced a decade ago. The present invention is believed to contribute to the current developments by providing a pointer/tracker control system, which allows multiple independent optical telescopes to be used as a phased array in a laser transmitter and which causes aIl telescopes to be steered together.